1. Field of the Invention
The present invention relates to a circuit arrangement and control method for generating high frequency sinusoidal voltage for igniting a high intensity discharge (HID) lamp, and for maintaining stable lamp arc operation with minimal high frequency ripple superimposed on a low frequency rectangular lamp current.
2. Description of Background Information
In electronic high intensity discharge lamp ballasts, there are two distinctly different methods to drive the lamp. The first method is to drive the lamp with high frequency sinusoidal current, and the second is to drive the lamp with low frequency rectangular current. The high frequency sinusoidal current method tends to give rise to acoustic resonance. Accordingly, low frequency rectangular current wave operation remains the favored technique for electronic high intensity discharge lamp ballasts because of the acoustic resonance problem associated with the high frequency method.
Two fundamental approaches are generally taken to generate a low frequency (less than 1 KHz) rectangular current with very small high frequency ripple to the lamp, as shown in comparative examples in FIGS. 1 and 2. FIG. 1 shows a buck power regulator with pulse ignition in discontinuous inductor current mode, including switching element Q1a, inductor L1a, diode D1a and capacitor C1a. In this case, the current in the inductor L1a has very large triangular high frequency ripple.
U.S. Pat. No. 5,428,268 to Melis et al., issued Jun. 27, 1995, describes one implementation substantially corresponding to the example of FIG. 1. As shown in FIG. 5A of Melis et al., the average part of the inductor current goes to the lamp, while the AC part of the inductor current is filtered by a capacitor C20 across the lamp. The patent to Melis et al. includes no specific mention of the actual values of capacitance C20 and inductance L20. However, to sufficiently filter the AC high frequency current to be below an acceptable level, and to maintain discontinuous mode operation for switching efficiency, the capacitance C20 has to be very large and the inductance L20 has to be quite small. The characteristic impedance of the circuit is low because of the large value of capacitance C20 and small value of inductance L20. It is known that the resonant voltage can be approximated by the characteristic impedance multiplied by the resonant current. Accordingly, generation of a high ignition voltage using the resonant method necessarily suffers from high circulating resonant current in the resonant elements and driving source switches. Practically, for an example of C=0.47 .mu.F, L=890 .mu.H, and Vp=3 kV.sub.peak, the resonant current will be an implausible 69 A.sub.peak, Obviously, the pulse method as disclosed in Melis et al. is the only logical method to ignite the lamp for the circuit arrangement and for the mode of operation disclosed therein.
The disadvantages of pulse mode ignition are clearly explained in commonly assigned U.S. patent application Ser. No. 08/783,557, filed Jan. 14, 1997, inventor(s) Ajay Maheshwari et al. FIG. 2 of the present application shows a comparative example of a buck power regulator with high frequency resonant ignition, similar to that of U.S. application Ser. No. 08/783,557, and with continuous inductor current mode. Shown in FIG. 2 are switches Q1b, Q2b; diodes D1b-D4b, inductor L1b, and capacitors C1b, Ca.sub.b and Cb.sub.b. In this case, the current in the inductor L1b has a very small triangular high frequency ripple superimposed on the low frequency rectangular current. Both the average part of the inductor current and the AC part of the inductor current flow through the lamp LMP. The parallel capacitor C1b with small capacitance is present only for the purpose of generating ignition voltage, and the burden of filtering the high frequency ripple is almost entirely on the inductor L1b. The disadvantages of this arrangement become apparent when it is considered that the high frequency attenuation is only -20 dB/decade (logarithmic decade) for frequencies above the corner frequency (the corner frequency being formed by the lamp LMP impedance and the inductance L1b). To achieve ripples low enough to avoid any acoustic resonance problems, the physical size of the inductor L1b, and the inductance itself, must be fairly large. A side effect of large inductance is an increased glow-to-arc transition time. Another disadvantage of this arrangement is that the switching elements Q1b, Q2b are in hard switching mode during the switch turn-on interval. The necessary switches are expensive because external ultra-fast freewheeling diodes in the order of 20-50 nS reverse recovery time are required. Moreover, switching losses are relatively high.
U.S. Pat. No. 4,904,907 to Allison et al., issued Feb. 27, 1990, discloses a modification of the continuous mode operation discussed above, in which (as shown in FIG. 5 of Allison et al.) an LC parallel resonant network (part of T301 and C304, C305 combination) is inserted into the buck inductor (part of T301). The inserted LC parallel resonant network has a resonant frequency at the buck operating frequency, and the fundamental frequency of the buck power regulator is attenuated significantly. A drawback of the circuit of Allison et al. is that the attenuation factor is highly sensitive to the frequency variation of the buck converter.
For example, the impedance of an LC parallel network can be calculated as ##EQU1## where wp is the parallel LC resonant frequency. The impedance at 1% and 3% deviation from the resonant frequency is Z.sub.p (1.01 wp)=50.2 and Z.sub.p (1.03 wp)=16.9, respectively. It can been seen that a 2-percentage point variation in the operating frequency will cause the attenuation impedance to vary by a factor of 3, which in turn will cause the high frequency ripple to be attenuated by almost the same factor.
In the above mentioned two patent disclosures (U.S. Pat. Nos. 5,428,268 and 4,904,907), two stages of conversion are required to regulate the power and to supply a rectangular current to the lamp. The first stage regulates the lamp power and limits the current in the lamp during warm-up phase. The high frequency ripple is also attenuated by the filters in the first stage. The second stage is a full bridge inverter that takes the DC output from the buck regulator and converts the DC output to a low frequency rectangular current (AC) for the lamp. A pulse ignition circuit is invariably required to ignite the lamp.
U.S. Pat. No. 4,912,374 to Nagase et al., issued Mar. 27, 1990, discloses a high frequency resonant ignition technique, although such is not the primary subject matter of this patent and is not specifically mentioned therein. In this topology, e.g., FIGS. 1 and 3 of Nagase et al., the power control stage and the inverter stage are combined in a half bridge/full bridge topology. The power control stage is combined with the output inverter, and in order to prevent acoustic resonance, the output inductor L1 and the capacitor C1 across the lamp must provide sufficient filtering to keep the high frequency component of the lamp current to a minimum. Consequently, the capacitance C1 is large, in the order of 1/10 micro-farads (.mu.F). When this scheme is operated at high frequency and the lamp is off, the resonant circuit formed by the inductor and capacitor produces a high voltage to ignite the lamp. Very large circulating current flows in the circuit because of the large capacitance value and the relatively smaller inductance value. When the lamp is in high frequency operation, high frequency current is produced in the lamp. During the low frequency mode, the switching pattern is changed to one that would control the lamp power and limit the lamp current. Fundamentally, the disclosure of Nagase et al. has the same disadvantages as the comparative example of FIG. 1 of the present disclosure in discontinuous mode operation, except that resonant ignition is implied.